1. Field of the Invention
The present invention is related to pulse doppler radar for detecting, precisely locating and tracking moving objects including airborne and spaceborne objects and, more particularly, to airborne or spaceborne pulse doppler radar for detecting, precisely locating and tracking moving objects on the ground.
2. Background Description
A monostatic radar system has its transmitter and receiver at the same location. A bistatic radar has a transmitter at one location and a receiver at a different, most often, distant location. Regardless of whether a radar system is monostatic or bistatic, when a target is detected using a single radar sensor, the target""s positional range (distance from the sensor) is highly accurate. However, the target""s cross-range (also known as its azimuthal position or sometimes, doppler direction) is much less accurate. Thus, a target""s distance is always known to a much greater degree of certainty than the target""s actual position. Coarser, azimuthal accuracy is due largely to radar beam spreading, which occurs in the beam both from the radar transmitter to the target and from the target to the sensor, i.e., the receiver.
Typically for a moving target, azimuthal position uncertainty is between xc2xc to {fraction (1/10)} of the beam width and depends upon the relative strength of the radar target""s return signal. Invariably, this azimuthal position uncertainty is much larger than the range or down-range positional uncertainty and, almost invariably, much larger than the desired level of uncertainty. Accordingly, the actual position of a target on the ground lies within an elliptically shaped area or on a spheroid that is identified by the positional inaccuracies, i.e., the azimuthal uncertainty distance defining major axis and the range uncertainty defining the minor axis.
For example, an airborne ground-moving-target surveillance radar at a standoff distances of 100 miles with a nominal pulse width of 200 MHz has a range positional uncertainty of approximately 1 meter. By contrast, the azimuthal or cross-range positional uncertainty may be tens of meters. Further, this azimuthal positional uncertainty is even more exaggerated for space based radar. The range positional uncertainty for the single detection of a moving target from a xe2x80x9ctypicalxe2x80x9d feasible space based radar is at least as good as airborne radar, i.e., as little as 1 meter or better. However, the azimuthal position uncertainty for space based radar may be on the order of hundreds of meters and even a few kilometers.
Operating two radar systems simultaneously at platforms positioned at different angles with respect to the target improves azimuthal positional accuracy. Radar sensors detect targets with a position-error ellipse with a high degree of ellipticity, i.e., the ratio of major axis to minor axis is hundreds to one or even a thousand or more to one because of the previously stated disparity in accuracy between range measurements and cross-range measurements. Thus, even a small difference in viewing angle of a moving target from two radar sensor platforms drastically reduces the target""s position uncertainty. For example, at best case where the two systems"" sensors are at a right angle to each other with respect to the target, the target""s positional uncertainty within in the plane defined by the target and the two platforms is bounded by the range positional uncertainty for the target from either sensor. Thus, the dual sensor target positional uncertainty is dramatically reduced as a result of combining the range position information from both sensors over that of independent sensors acting alone.
However, it is well known that difficulties arise even using two radar sensors. It is especially difficult to locate and track one target when the target density of moving targets in an area being surveilled exceeds or becomes comparable to the major axis dimension (azimuthal dimension) of a single sensor error ellipse. When this occurs, multiple targets occur at nearly the same range from each sensor, causing target error ellipses from one sensor to overlap target error ellipses from the other sensor at multiple locations. This makes accurately locating and tracking targets much more difficult. While some of these overlapping error ellipses correspond to true target locations, some overlapping occurs from other different targets. This second type of overlap can result in a false target or a xe2x80x9cghostxe2x80x9d and is typically referred to as a multi-lateration problem. It is difficult to discriminate between a true targets and ghosts under these conditions.
These large cross range positional errors are vexations to current radar systems as well as multi-target indication surveillance systems currently being developed, whether they are intended as airborne or spaceborne radar systems. Ideally, targets of interest are tracked for as long as tens of minutes. Unfortunately, with cross range errors that may be as much as tens or hundreds of meters or even kilometers, even using bistatic radar this tracking capability is not generally possible, especially where the target of interest is near other moving objects, e.g., a truck on a busy road. Additionally, radar surveillance systems for weapons delivery platforms must be able to make a very precise handoff to, for example, a manned aircraft or even a GPS guided weapon. This precision is not available with current state-of-the-art airborne target surveillance radars, much less for spaceborne moving target surveillance radars.
Thus, there is a need for a way to resolve target position ambiguities or ghosts in pulse doppler radar systems. There is a further need for a radar system that can track individual moving targets in close proximity to each other.
It is a purpose of the invention to improve radar system positional accuracy;
It is another purpose of the invention to track moving targets in close proximity to each other;
It is another purpose of the invention to resolve target position ambiguities (ghosts) in two sensor multi-lateration that can occur in pulse doppler radars especially at high moving target density.
The present invention is a pulse doppler radar system simultaneously operating monostatically and bistatically. A pair of radar units operate monostatically transmitting a radio frequency (RF) energy signal and receiving a RF return from the transmitted RF energy signal. In addition, simultaneously, one radar unit receives a bistatic return from the other unit. Information from the bistatic return is combined with information from both monostatic returns to locate individual targets.
In particular, for airborne or spaceborne pulse doppler radar, the present invention may be used for detecting, precisely locating, and tracking moving ground based objects. Further, the present invention has application to pulse doppler radar regardless of source/sensor location, used for detecting, precisely locating and tracking moving objects at any location, including, for example, ground, airborne and spaceborne objects.